- - - - - - .. 1\ 11\ 11111\ 111\ II \\11 11\111 \111 \1\\ II 11\\ 11\11\ II \11\ 111\\\ II I1 \ 3 1176 00166 1835 DOE/NASA/51 044-23 e NASA TM-82700 NASA-TM-82700 198 10024941 Continuously Variable Transmission-Assessment of Applicability to Advanced Electric Vehicles Stuart H. Loewenthal and Richard J. Parker National Aeronautics and Space Administration Lewis Research Center LIBRARY COpy OCT 2 9 1981 LANGLEY R"S7/\R.- , : £NTER UBP..t.RY, NASA HAMPTON, VIRGINIA Work performed for U.S. DEPARTMENT OF ENERGY Conservation and Renewable Energy Office of Vehicle and Engine R&D Prepared for Electric Vehicle Council Symposium VI Baltimore, Maryland, October 21 - 23, 1981 L ~, " • I I \ \ ~. NOTICE This report was prepared to document work sponsored by I the United StateS Government. Neither the United States nor its agent, the united States Department of Energy, nor any Federal employees, nor any of their contractors, subcontractors or their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or useful­ ness of any information, apparatus, product or process disclosed, or represents that its use woul~ not infringe privately owned rights. DOE/NASA/51 044-23 NASA TM-82700 ,F- Continuously Variabl~ Transmission-As'sessment of Applicability to Advanced Electric Vehicles Stuart H. Loewenthal and Richard J. Parker National Aeronautics and Space Administration Lewis Research Center Cleveland, Ohio 44135 Work performed for U.S. DEPARTMENT OF ENERGY Conservation and Renewable Energy Office of Vehicle and Engine R&D Washington, D.C. 20545 ;. Under Interagency Agreement DE-AI04-77CS51 044 Electric Vehicle Council Symposium VI Baltimore, Maryland, October 21-23, 1981 ~. t/<l1-J.3 $I ~~ CONTINUOUSLY VARIABLE TRANSMISSION - ASSESSMENT OF APPLICABILITY TO ADVANCED ELECTRIC VEHICLES by Stuart H. Loewenthal and Richard J. Parker National Aeronautics and Space Administration Lewis Research Center Cleveland, Ohio 44135 ABSTRACT A brief historical account of the evolution of continuously variable transmissions- (CVT) for automotive use is given. CVT concepts which are potentially suitable for application with electric and hybrid vehicles are discussed. The arrangement and function of several CVT concepts are cited 'along with their current developmental status. The results of preliminary design studies conducted under contract to NASA for DOE on four CVT concepts for use in advanced electric vehicles are discussed. For these studie~ a 1700 kg (3750 Ib) vehicle with an energy storage flywheel was specified. Requirements of the CVTs were a maximum torque of 450 N-m (330 lb-ft), a ~ maximum output power of 75 kW (100 hp), and a flywheel speed range of 14 000 ~ to 28 000 rpm. System life was to exceed 2600 hours at a 90-percent proba- W bility of survival. Efficiency, size, weight, cost, reliability, maintain­ ability, and controls were evaluated for each of the four concepts. The design studies were performed by Garrett/AiResearch (torodial traction CVT), Battelle Columbus Labs (steel V-belt CVT), Kumm Industries (flat belt vari­ able diameter pulley CVT) , and Bales-McCoin Tractionmatic (cone-roller trac­ tion CVI). All CVTs exhibited relatively high calculated efficiencies (86 to 97 percent) over a broad range of vehicle operating conditions. Esti­ ~ated weight and size of these transmissions were comparable to or les~ than an equivalent automatic transmission. INTRODUCTION The range of an electric vehicle is primarily dependent on the energy capacity of the batteries and the rate at which power is withdrawn. Electric vehicles have been constructed without multi-speed transmissions but the lack of torque multiplication that the transmission affords results in high current drains on the batteries during starting, passing and hill climbing. High discharge currents not only adversely affect battery capac­ ity and life but, moreover, require the use of unnecessa~ily large motors and more costly controls. Furthermore, using only voltage 'control with a D.C. motor results in less efficient motor operation over the vehicle's driving cycle than that attainable with a variable speed transmission. In a recent propulsion system design study (ref. 1). The addition of a multi­ ratio transmission significantly reduced propulsion system weight and lessened its cost for an electric vehicle powered by a DC shunt motor with field'control. Incorporating a continuously variable transmission (CVT) and flywheel energy storage device into this system resulted in the lightest of the 17 configurations investigated and the fourth least expensive (ref. 1). , ," Although CVTs offer potential p~rformance advantages, the bulk of transmissions currently used in electric vehicle are either one speed (direct coupled) or discrete multispeed units (ref. 2). Host of the multi­ speed units are small passenger car transmissions. In the case of cars con­ verted to run on electrical power, existing transmissions are usually left in place. In most cases, the size and speed ratios of these transmissions are not well matched to the operating characteristics of the electric motor and vehicle. The limited use of CVTs in passenger cars has hindered their applica­ tion to electric vehicles. As will be discussed, CVTs were quite popular with designers of early automobiles but this popularity was relatively short lived. Improvements in the shifting characteristics of manual gearboxes in the late 1920's lessened the incentive to develop improved continuously variable transmissions. Through the years there have been occasional attempts, both here and in Europe, to commercially introduce CVTs into passenger cars. Several of these efforts proved technically feasible, but they were never really serious contenders to replace the automatic, torque­ convertor, gear-shift transmission which was introduced in the early 1940's. Up until the 1970's, the primary emphasis for automatic transmis­ sions was on transmission shift quality and cost while efficiency was basi­ cally a secondary consideration. In recent years the emphasis has changed to improving passenger car fuel economy by improving drive train effi­ ciency. The shortage of petroleum has also stimulated research on alternate types of automotive powerplants, such as electric and gas turbine. Onboard flywheel energy storage devices are also being investigated as a means of improving fuel economy. These factors, in turn, have triggered renewed CVT activity. The eVT's "infinite" number of shift points offers the engine designer the greatest possible latitude in optimizing his drive train, no matter what the powerplant. It is the intent of this paper to review some of the ~ast and more recent CVT development activities, particularly those which are potentially suitable for electric and hybrid vehicle applications. A second objective is to review the results of preliminary design studies that have been re­ cently conducted on four CVT concepts for use with a flywheel equipped elec­ tric vehicle. Past Automotive CVTs Although the potential performance benefits associated with a contin­ ously variable transmission for various types of machinery had been re­ cognized at an early date, it wasn't until the-introduction of the horseless carriage at the end of the nineteenth century that the goa~ of developing a CVT took on real meaning. It quickly became apparent to the designers of early automobiles that a highly flexible transmission was badly needed to compensate for the lack of flexibility of "the early gasoline engines. These engines had a tendency to run well at only one speed. As noted by P. M. Heldt in an unusually comprehensive review of the development of the automatic transmission (ref. 3), the chief objection to early sliding-gea~ transmission, apart from their lack of flexibility was the difficulty in gear shifting. The gearboxes used on many of the early vintage cars, such as 1890 Panhard, were adopted from the clash gears used in factory equipment 2 (ref. 4). Gear changing was not merely difficult but potentially damaging to the gear teeth. According'to Hodgesafld'Wise (ref. 5), "the best tech­ nique with those early cars was to slip the clutch gently and bang the gears home as quickly as possible, in the hope that you might catch the cogs un­ awares." {Although Prentice and Shiels patented the syn~hromesh principle back in 1904, it wasn't until the late nineteen twenties that General Motors perfected the syncromesh gearshift for production which allowed almost any driver to shift from one speed to the next without clashing the gears (ref. 6». In view of the limitations and inconveniences associated with gear changing, it is not surprising that the inventors of the day looked for a simple means of continuously and, hopefully, automatically varying the speed ratio between the engine and the wheels. ' Mechanical ratchet, hydraulic and electro-mechanical drives were tried but drives using friction power transfer were the first mass-produced auto­ mobile transmissions to provide continuous ratio selection. The earliest of these were rubber V-belt drives appearing on the 1886 Benz and Daimler cars, the first mass-produced gasoline engine powered vehicles. Friction disk drives, similar in construction to the Gearless Transmission as illustrated in a 1906 advertisement shown in figure 1 (ref. 7) were used as regular equipment on a number of early motor cars. These included the 1906 Cartercar, ,1907 Lambert, 1909 Sears Motor Buggy and 1914 Metz Speedster. An early advertisement for the Lambert appears in figure 2. The Cartercar, pictured in figure 3, ,had an extremely simple friction drive consisting of a metal'disk driven by the engine crankshaft which was in traction contact with a large, fiber-covered spoked wheel mounted on a transverse countershaft. The countershaft, in turn, was connected to the rear axle by an ordinary chain drive. To vary speed ratio, a driver operated lever (see fig. 3) was used to radially position the output follower wheel across the face of the metal disk; turntable fashion. Neutral was achieved when the follower wheel was centered.